The Coriolis flowmeter is a mass flowmeter, which utilizes the fact that, when a tube through which a fluid to be measured flows is supported at one end or both ends thereof, and vibration is applied to a portion of the tube around the supporting point in a direction perpendicular to the flowing direction of the tube, the Coriolis forces applied to the tube (hereinafter, a tube to which vibration is applied is referred to as flow tube) are proportional to a mass flow rate. The Coriolis flowmeter, which is well known, is roughly classified into a straight tube type and a curved tube type in terms of the flow tube shape of the Coriolis flowmeter.
The Coriolis flowmeter is a mass flowmeter for detecting a phase difference signal proportional to a mass flow rate at symmetrical positions between support portions for both ends and a central portion of a measurement tube through which a fluid to be measured flows in a case where the measurement tube is supported at both ends thereof and the central portion of the supported measurement tube is alternately driven in a direction perpendicular to a support line. The phase difference signal is a quantity proportional to the mass flow rate. When a driving frequency is maintained constant, the phase difference signal may be detected as a time difference signal at the observation positions of the measurement tube.
When the alternate driving frequency of the measurement tube is made equal to the natural frequency of the measurement tube, a constant driving frequency corresponding to a density of the fluid to be measured is obtained, and hence the measurement tube may be driven with small driving energy. Therefore, recently, the measurement tube has generally been driven at the natural frequency and the phase difference signal is detected as the time difference signal.
In the straight tube type Coriolis flowmeter, when vibration is applied to a straight tube having both ends supported, in a direction perpendicular to the straight-tube center portion axis, a difference in displacement of the straight tube due to the Coriolis forces is generated between the support portions and the central portion of the straight tube, that is, the phase difference signal is obtained, and, based on this phase difference signal, the mass flow rate is detected. The straight tube type Coriolis flowmeter thus constructed has a simple, compact, and solid structure. On the other hand, there arises a problem in that high detection sensitivity cannot be achieved.
In contrast, the curved tube type Coriolis flowmeter is superior to the straight tube type Coriolis flowmeter in that the curved tube type Coriolis flowmeter allows selection of a shape for effectively obtaining the Coriolis forces. In fact, the curved tube type Coriolis flowmeter is capable of performing mass flow rate detection with high sensitivity. Known examples of the curved tube type Coriolis flowmeter include one including a single flow tube (see, for example, JP 04-55250 B), one including two flow tubes arranged in parallel (see, for example, JP 2939242 B), and one including a single flow tube in a looped state (see, for example, JP 2951651 B).
Incidentally, a combination of a coil and a magnet is generally used as driving means for driving the flow tube. The coil and the magnet are preferably attached to positions which are not offset in the vibration direction of the flow tube because a positional relationship deviation between the coil and the magnet is minimized. Therefore, the two flow tubes arranged in parallel as disclosed in JP 2939242 B are attached so as to sandwich the coil and the magnet. Therefore, the Coriolis flowmeter is designed so that the two opposed flow tubes are separated from each other at least an interval to sandwich the coil and the magnet.
Of Coriolis flowmeters including two flow tubes located in parallel planes, a Coriolis flowmeter having a large diameter or a Coriolis flowmeter having high flow tube rigidity is required to increase power of the driving means, and hence it is necessary to sandwich large driving means between the two flow tubes. Therefore, such a Coriolis flowmeter is designed so that an interval between the flow tubes is necessarily widened even in a fixed end portion which is a base portion of the flow tubes.
As illustrated in FIG. 6, a general Coriolis flowmeter 1 includes a detector 4 and a converter 5 for two U-shaped tubes 2 and 3.
A vibrator 6, velocity sensors 7, and a temperature sensor 8 are attached to the detector 4 for the measurement tubes 2 and 3 and connected to the converter 5.
The converter 5 of the Coriolis flowmeter includes a phase measurement section 11, a temperature measurement section 12, and a drive control section 13.
The phase measurement section 11 is configured as follows.
When digital signal processing is to be executed, the phase measurement 11 of the Coriolis flowmeter performs A/D conversion on signals from the pair of velocity sensors for digital conversion processing, and then obtains a phase difference between the converted signals.
Next, a measurement method used by the temperature measurement section 12 is described.
A temperature sensor for tube temperature compensation is provided in the Coriolis flowmeter.
A resistance type temperature sensor is generally used to measure a resistance value, to thereby calculate a temperature.
The drive control section 13 sends a predetermined mode signal to the vibrator 6 attached to the measurement tubes so as to enable the tubes 2 and 3 to resonance-vibrate.
A conventional analog drive circuit having a positive feedback loop structure is frequently used, in which circuit components are different depending on a tube shape or the like, and hence it is difficult to obtain a common converter structure.
The drive circuit has the structure independent of the phase measurement section 12, and thus cannot control a performance function and is incorporated as a component of measurement means based on the Coriolis force principle (phase measurement). Therefore, the fact is that the drive circuit is not utilized for superior function deployment.
The conventional drive circuit has an analog circuit structure. The conventional drive circuit has a structure as illustrated in FIG. 7.
The operation principle of the drive circuit illustrated in FIG. 7 is described.
An input signal of a pick-off is full-wave rectified by a full-wave rectifier circuit 21 included in an amplitude measurement section 20. The input signal of the pick-off which is full-wave rectified by the full-wave rectifier circuit 21 is input to a low-pass filter 22 included in the amplitude measurement section 20.
In this way, the amplitude measurement section 20 obtains an amplitude value of an input waveform of the input signal of the pick-off which is input to the low-pass filter 22.
The amplitude value obtained by the amplitude measurement section 20 is subtracted by an adder 23 from a reference voltage value Vref input to the adder 23, and then multiplied by a multiplier 24 by the input signal of the pick-off which is input to the amplitude measurement section 20. An input signal obtained by multiplying by the multiplier 24 is input to a drive output amplifier 25, and then output as a drive signal from the drive output amplifier 25.
When the amplitude value of the input signal does not reach a predetermined level, an output of a start circuit 26 is changed to switch a gain of the output amplifier 25. Then, a level of the drive signal increases, and hence the input signal rapidly converges to the predetermined level.
The conventional drive circuit operating as described above has the merit that the response to the change in input signal is excellent because the drive circuit has the analog circuit structure, but has the following demerits.